1. Field of the Invention
The present invention relates to a blood-pressure monitoring apparatus which monitors a blood pressure of a living subject.
2. Related Art Statement
As information relating to a pulse wave which propagates through an arterial vessel of a living subject, there are known a pulse-wave-propagation time DT and a pulse-wave-propagation velocity V.sub.M (m/s). The pulse-wave-propagation time DT is a time which is needed by a pulse wave to propagate between two different positions of the arterial vessel. Additionally, it is known that the above pulse-wave-propagation-relating information is, within a predetermined range, substantially proportional to the blood pressure ("BP", mmHg) of the living subject. Therefore, there has been proposed a BP monitoring apparatus which determines, in advance, coefficients a, B in the following expression: EBP=.alpha.(DT)+.beta. (where .alpha. is a negative constant and .beta. is a positive constant), or EBP=.alpha.(V.sub.M)+.beta. (where a and .beta. are positive constants), based on two measured BP values of the subject and two measured pulse-wave-propagation time values (DT) or two measured pulse-wave-propagation velocity values (V.sub.M), iteratively determines an estimated BP value EBP of the subject, based on each set of subsequently obtained pulse-wave-propagation-relating information, according to the above-indicated first or second expression, and starts a BP measurement using an inflatable cuff when an abnormal estimated BP value EBP is determined. An example of the BP monitoring apparatus is disclosed in U.S. Pat. No. 5,752,920.
Meanwhile, the elasticity or flexibility of blood vessels of a patient as a living subject may be lost due to arteriosclerosis or temporary constriction of the blood vessels. In this case, the BP of the patient cannot be easily controlled. Therefore, it is needed to quickly measure a BP value of the patient and quickly make a decision about whether or not any treatments should be given to the patient. This is also the case with a patient who has fallen in shock because of excessive expansion of the blood vessels upon administration of a hypotensive drug.
In addition, a relationship between pulse-wave-propagation-relating information and estimated BP value EBP changes depending upon a hardness of a blood vessel for which the pulse-wave-propagation-relating information is obtained. That is, the coefficient .alpha. of each of the above-indicated two expressions that defines the relationship between pulse-wave-propagation-relating information and estimated BP value EBP changes depending upon the hardness of the blood vessel. Therefore, if the hardness of the blood vessel largely changes while estimated BP values EBP are iteratively determined for monitoring the BP of the patient, the accuracy of the estimated BP values EBP decreases, which may lead to delaying commencement of a BP measurement using the inflatable cuff.